The present invention relates to an automatic transmission for a vehicle in which multiple speeds or a wide gear ratio range are required.
Conventionally, there have been proposed various multiple-speed transmissions that realize eight forward speeds using three planetary gearsets and six friction elements. Japanese Patent Application Provisional Publication No. 2001-182785 discloses such an eight forward-speed transmission that includes a double-pinion planetary gearset, a Ravigneaux planetary gear unit (a combination of a double-pinion planetary gearset and a single pinion planetary gearset), four clutches, and two brakes.
However, in the above conventional art, the following problems are caused due to the two double-pinion planetary gearsets used.    (1) The number of meshing engagement between gears is increased to thereby cause deterioration in gear efficiency and gear noise.    (2) A diameter of the pinion gears is reduced to thereby lower endurance reliability of the transmission.    (3) The number of parts in the transmission is increased to thereby cause an increase in cost.
In addition, in the above conventional art, it is required that two of the six friction elements are in an engagement state in order to establish the eight forward speeds. Due to this construction, the remaining four friction elements are idle at the respective speeds. This results in increase in friction loss in the idling friction elements, thereby causing deterioration in driving energy transmission efficiency.
The present invention has been made in view of the above problems. It is an object of the present invention to provide an automatic transmission that is advantageous in view of gear efficiency, reduction of gear noise, endurance reliability, and cost performance and is capable of increasing driving energy transmission efficiency by reducing friction loss in friction elements.
In one aspect of the present invention, there is provided an automatic transmission that includes an input shaft, an output shaft, a first planetary gearset including a first sun gear, a first pinion carrier that supports a first pinion gear meshing with the first sun gear, and a first ring gear meshing with the first pinion gear, a second planetary gearset including a second sun gear, a second pinion carrier that supports a second pinion gear meshing with the second sun gear, and a second ring gear meshing with the second pinion gear, a third planetary gearset including a third sun gear, a third pinion carrier that supports a third pinion gear meshing with the third sun gear, and a third ring gear meshing with the third pinion gear, and six friction elements, the automatic transmission being capable of establishing at least eight forward speeds by suitably engaging and disengaging the six friction elements and outputting a torque from the input shaft to the output shaft,                wherein the input shaft is always connected to the first sun gear,        the output shaft is always connected to the second ring gear,        the third sun gear is always kept in a fixed state relative to a transmission case of the automatic transmission,        the first ring gear and the third pinion carrier are always connected to each other and constitute a first rotary member,        the six friction elements include a first friction element that selectively connects the first sun gear and the second pinion carrier, a second friction element that selectively connects the first pinion carrier and the second sun gear, a third friction element that selectively connects the first pinion carrier and the second pinion carrier, a fourth friction element that selectively connects the second sun gear and the first rotary member, a fifth friction element that selectively connects the second sun gear and the third ring gear, and a sixth friction element that is operative to interrupt rotation of the second pinion carrier, and        at least eight forward speeds and one reverse speed are respectively established by simultaneous engagements of three friction elements selected from the six friction elements.        